It is of course well-known to sterilise metal surgical instruments by steam and it has also been common practice to sterilise various articles in sterilising liquids. High temperature steam sterilising is simple and effective for certain metal objects, but has limitations and disadvantages, for example it is inefficient in sterilising any object within a sealed package, and the high temperatures involved may cause degradation of plastic articles. The use of sterilising liquids also has limitations, for example air bubbles may be trapped within the object thus preventing full effectiveness, and it may be difficult or impossible to remove all trace of the liquid afterwards.
It has also been common practice to sterilise medical or surgical equipment by means of a toxic gas and many existing sterilisers in hospital use, for example, involve contacting the articles with Ethylene Oxide in a sealed sterilising chamber. Ethylene Oxide is an extremely effective bactericide, and is effective not only for metal objects such as surgical implements, but also relatively delicate instruments and equipment, including synthetic plastics. The gas is effective at medium or low temperatures and thus the effects of high temperatures on degrading materials can be avoided. The gas penetrates or permeates plastic wrappings and can thus be used to sterilise pre-packaged objects.
Ethylene Oxide is, however, extremely toxic to human beings and after its use for sterilising it is important that steps be taken to remove all traces, as far as possible. To this end it is known to flush out the interior of the sterilising chamber or autoclave with fresh air after the sterilising gas has been extracted. It, has also been proposed to "pulse" or vary the pressure of the flushing air during the flushing operation.
One of the problems involved is that the Ethylene Oxide is absorbed into a number of typical plastic materials which may need to be sterilised, and to "desorb" the gas from the plastics appears to be extremely difficult. Even after the sterilising chamber has been flushed the desorption of the gas from the objects may result in contaminating the atmosphere of the laboratory or other workroom to a concentration of, say, 5 ppm or more which is an unacceptable health hazard.